Many of the stem cells being studied are referred to as pluripotent, meaning they can give rise to any of the cell types in the body but they cannot give rise on their own to an entirely new body. (Only the earliest embryonic cells, which occur just after fertilization, can give rise to a whole other organism by themselves.) Other stem cells, such as the ones found in the adult body, are multipotent, meaning they can develop into a limited number of different tissue types.

One of the most common stem cell treatments being studied is a procedure that extracts a few stem cells from a person's body and grows them in large quantities in the laboratory—what scientists refer to as expanding the number of stem cells. Once a sufficient number have been produced in this manner, the investigators inject them back into the patient.

The bone marrow is a rich source of adult stem cells, containing both the hematopoietic stem cells that give rise to the various types of blood and the so-called mesenchymal cells, which can develop into bone, cartilage and fat. Mesenchymal cells are found in the bone marrow and various other places in the body, although whether all mesenchymal stem cells are truly interchangeable irrespective of origin is unclear.

Scientific American spoke with Mahendra Rao, director of the Center for Regenerative Medicine at the National Institutes of Health in Bethesda, Md., to get a sense of the sorts of new developments that might occur in regenerative medicine in the next five years or so.

[An edited transcript of the interview follows.]

Why is there so much excitement about regenerative medicine?
You could say that medicine up until now has been all about replacements. If your heart valve isn't working, you replace it with another valve, say from a pig. With regenerative medicine, you're treating the cause and using your own cells to perform the replacement. The hope is that by regenerating the tissue, you're causing the repairs to grow so that it's like normal.

And research into regenerative medicine has been going at a pretty fast pace.
Yes, there have been a lot of novel breakthroughs in the past few years. It seems that things are moving relatively rapidly to true translation and clinical practice. When you think about it, it usually takes 10 to 14 years to bring a drug to market. Well, the entire field of pluripotent stem cells is 14 or 15 years old. And yet, in those 15 years there are already clinical trials in place.

And if you include the non-pluripotent cells, then there are already five or six products in the U.S. and other countries. There's Provenge [for prostate cancer], Appligraf [to treat diabetic foot ulcers], Carticel [to replace knee cartilage], Gintuit [to promote healing after gum surgery] and Fibrocell is replacing fibroblasts.

Of course, all of these are still small. But what they say is that the principle works—and because the principle works, then bigger things can be tackled.

What's next for regenerative medicine?
In my mind there are four [upcoming] areas for regenerative medicine: all things related to the mesenchymal stem cells; neural cells; the eye; and the last bucket—building in three dimensions, using a mixture of cells and structures.

What’s so special about mesenchymal cells?
They can expand a lot. They're found in sufficient amounts in the adult so that you can treat patients with their own cells—and avoid having to give powerful antirejection medication. Mesenchymal cells can be used, for example, to treat the heart, the blood vessels and Crohn's disease.

What's exciting about the neural cells is that this is the first that we've had access to cells from the central nervous system in the numbers that allow us to think about potential treatments.

As for the eye, we can do a lot of things in the eye now, and it’s been quite exciting. There have been several companies and clinical trials trying different things.

Such as?
Several people are moving forward with treatments for both the wet and the dry macular degeneration. It’s new technology that's potentially disruptive.

And the fourth bucket?
The fourth bucket is like a mixture. The excitement here has been that we could always try to make three-dimensional tissue but it was never very good. But somebody had the idea that if we combined the structures with the cells, the cells would build the three-dimensional tissue for us.

Two big things I didn’t put in here: One is the heart and one is diabetes. Both are of huge interest. The only reason I didn’t put them higher on the list is that they are more like five to seven years away. The four buckets I just mentioned are less than five years off.